1. Field
Various embodiments of the invention pertain to wireless communication systems. At least one embodiment of the invention pertains to a system and method for Medium Access Control (MAC) of a wireless communication system.
2. Background
Code Division Multiple Access (CDMA) is a wireless communication technique used by many modern communication systems. CDMA is a spread-spectrum communication protocol that utilizes coded signals occupying the same spectral bandwidth (e.g., 1.25 MHz). Typically, the orthogonally coded signals are used on the forward link (e.g., base stations to user terminals) while coded signals of arbitrary orthogonality are used on the reverse link. A CDMA signal is spread by one of N (e.g. N=64) codes (e.g., Walsch codes or “spreading codes”) that spread the signal over a bandwidth range (e.g., approximately 1.25 MHz). In order to allow several users to simultaneously share the same frequency band, the user Access Terminals (AT) employ different or random codes to spread their transmitted waveforms. Such a multiplexing strategy results in what is known as an “interference shared” link. Two important characteristics of this type of link are that (1) the different AT transmissions interfere with each other and (2) the aggregate received power at the base station antenna(s) is tied to the stability and coverage of the communications system. As the aggregate received power increases, both the stability and coverage of the communication system worsen.
Since individual channel conditions and the total number of users present in a communication system can vary with time, a set of control algorithms is used to ensure that the maximum number of users have communication links that meet their grade of service requirements while the total amount of power received remains under a predetermined level. Such algorithms are typically known as Medium Access Control (MAC) algorithms and regulate the “sharing” of the aggregate power among the ATs.
One problem in designing such MAC schemes is the partitioning of a total resource between many users while assuring certain levels of performance for the ATs communication links. Effective MAC algorithms typically rely on some feedback between the Access Network (AN), which typically includes one or more base stations, and the ATs. One typical method of control feedback is the Power Control (PC) loop by which a base station can alter an AT's transmit power level through power control UP/DOWN commands sent on the forward link. As used herein, the “forward link” is the communication link from a base station to an AT and the “reverse link” is the communication link from the AT to the base station. The simplicity of the service requirements on early CDMA systems, where users would typically transmit the same type of traffic (voice) using the same data rate, allowed for relatively straightforward MAC algorithms. Since then however, more features have been introduced in response to the demand for extending the type of service beyond the constant bit rate and low latency “voice-like” traffic. For instance, support for discontinuous transmission, variable data rates and different types of traffic with different grade of service requirements. The reverse link of IS-856 Revision A [1] is a good example of such enhanced systems.
Supporting these new features has a significant effect on the complexity and effectiveness of the MAC algorithms, making it particularly difficult for the AN to control the behavior of each individual AT.
In conventional MAC algorithms, centralized congestion control is done on a sector-by-sector basis. A Reverse Activity Bit (RAB) is set and broadcasted by the AN and monitored by all ATs communicating through the AN. When turned On, the RAB indicates congestion and affects the data rate determination strategy of the ATs. Link monitoring is done by controlling the power for all users so that their frame error rate achieves some desired level, e.g., typically one percent or less. Normally, the data rate is decided by the AT in accordance with policies for increasing and decreasing the allowed rate which depend on the RAB. For instance, if the RAB is turned On, an AT is less likely to increase its data rate and more likely to decrease it. This type of congestion control is used to centrally control all ATs equally. However, this lack of individual congestion control limits flexibility when designing a policy for increasing the data rates. The inability of the AN to quickly address an individual AT, which may be taking up too much of the interference share (i.e., the AT's transmission power is too high), forces the system to apply stringent rules for allowing a rate increase.